prelu belongs in FP16_CASTS (#257)

The main use of these functions (e.g.: `torch.{conv*, prelu}`) is via their `torch.nn`
wrapping layers.

The `torch.nn` layers are what contain the weights and call into these lower level
functions with the weights as a parameter in their `forward()` method.

The `torch.conv*` functions are already in the `FP16_CASTS` list due to amp's philosophy of
casting the parameters rather than the model/layer weights.

Conceptually `torch.prelu` is the same as the `torch.conv*` case, where its weight parameter
is passed in from its wrapper layer `torch.nn.PReLU`.

apex/amp/lists/torch_overrides.py

@@ -5,8 +5,9 @@ from .. import utils
 MODULE = torch
 
 FP16_FUNCS = [
-    # Math
-    # TODO: why are these in top-level torch namespace?
+    # Low level functions wrapped by torch.nn layers.
+    # The wrapper layers contain the weights which are then passed in as a parameter
+    # to these functions.
     'conv1d',
     'conv2d',
     'conv3d',
@@ -14,6 +15,7 @@ FP16_FUNCS = [
     'conv_transpose2d',
     'conv_transpose3d',
     'conv_tbc',
+    'prelu',
 
     # BLAS
     'addmm',
@@ -76,7 +78,6 @@ CASTS = [
     'addcmul',
     'atan2',
     'cross',
-    'prelu',
 
     # Element-wise _or_ tensor-wise math
     'add',